You can't always justify dropping a diesel into an older rig with just fuel numbers. You get the benefits or better low-end power, more mileage per tank and you'll be able to do many trail runs without taking any extra fuel where you may have taken 5-10 extra in gas.

When dealing with a new vehicle, IMO, it is very easy to justify the extra purchase price (if you have that kind of extra changing in your piggy back) because the "diesel option" is generally about how much more the vehicle will be worth even when used. In other words, you'll get your money back.

Of course, you could also be a dork like my cousin and sink over $24K in your Cummins for pulling at the county fairs. Of course, he's almost 27 and still living with mom and dad.